At the outset, it should be noted that statements made herein merely provide information related to the present disclosure and do not constitute any admission of prior art.
In the oil and gas industry, a subterranean formation (i.e. a “reservoir”) often needs to be treated (or “stimulated”) to enhance or restore the productivity of a well. Generally speaking, stimulation treatments fall into two main groups, hydraulic fracturing treatments and matrix treatments. Fracturing treatments are performed above the fracture pressure of the reservoir formation and create a highly conductive flow path between the reservoir and the wellbore. Matrix treatments are performed below the reservoir fracture pressure and generally are designed to restore the natural permeability of the reservoir following damage to the near-wellbore area.
A commonly used method to treat the matrix is “matrix acidizing”, which is generally understood in the industry to mean the treatment of a reservoir formation with a stimulation fluid containing a reactive acid. In sandstone formations, the acid reacts with the soluble substances in the formation matrix to enlarge the pore spaces. In carbonate formations, the acid dissolves the entire formation matrix. In each case, the matrix acidizing treatment improves the formation permeability to enable enhanced production of reservoir fluids. Matrix acidizing operations are ideally performed at high rate, but at treatment pressures below the fracture pressure of the formation. This enables the acid to penetrate the formation and extend the depth of treatment while avoiding damage to the reservoir formation.
Typically, the reactive acid comprises hydrochloric acid (HCl) and a blend of acid additives. It is also common for acid treatments to include a range of acid types or blends, such as acetic, formic, hydrochloric, hydrofluoric and fluroboric acids. A few examples are set forth in commonly assigned U.S. Pat. Nos. 6,350,721, 6,828,280, 6,938,693, 7,306,041, 7,066,260, 7,318,475, the contents of which are hereby incorporated by reference into the current application in their entireties.
Depending on the characteristics of the formation and the treatment fluid, it may be helpful to first emulsify the acid before pumping it down the wellbore. The preparation of acid emulsion is traditionally performed off-site, i.e. at a location that is away from the wellsite, and is generally based on a batch mixing method. An example of the prior art system 100 is shown in FIG. 1. There, a large tank (not shown) is employed to re-circulate an acid mixture until a complete homogenous state is achieved. The emulsifying agent is transferred into a batch tank 140 and then the blended acid is added to the batch tank 140. A centrifugal pump 180 (i.e. c-pump) is then used to re-circulate the tank 140 until the desired emulsion is created. Once the emulsion is created, the contents of the batch tank 140 can be delivered to the wellsite as a finished product.
Batch mixing is disadvantageous for several reasons. First, it is often challenging to plan the logistics such as storage and transportation beforehand to ensure sufficient acid is available at the wellsite when an acid treatment needs to be performed on a wellbore. Second, batch-to-batch variations may occur, which could result in inconsistencies in treatment result. Third, degradations may occur during the process of storage and transportation. Fourth, while batch mixing a small volume of acid emulsions is not a significant issue, it can become very difficult when the volumes needed is very large. Accordingly, there is a need to improve the current batch mixing method for preparing acid emulsions.